• Deborah A. Clark,

    1. Department of Biology, University of Missouri-St. Louis; Mailing address: INTERLINK-341, P.O. Box 02-5635, Miami, Florida 33102 USA
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  • Sandra Brown,

    1. Department of Natural Resources and Environmental Sciences, University of Illinois, W503 Turner Hall, Urbana, Illinois 61801 USA
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    • Present address: Winrock International, 1611 N. Kent Street, Suite 600, Arlington, Virginia 22209 USA.

  • David W. Kicklighter,

    1. The Ecosystems Center, Marine Biological Laboratory, Woods Hole, Massachusetts 02543 USA
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  • Jeffrey Q. Chambers,

    1. National Center for Ecological Analysis and Synthesis, University of California, Santa Barbara, California 93101-3351 USA
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  • John R. Thomlinson,

    1. Institute for Tropical Ecosystem Studies, University of Puerto Rico, P.O. Box 363682, San Juan, Puerto Rico 00936-3682 USA
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  • Jian Ni

    1. Laboratory of Quantitative Vegetation Ecology, Institute of Botany, Chinese Academy of Sciences, Xiangshan Nanxincun 20, 100093 Beijing, P.R. China
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There are pressing reasons for developing a better understanding of net primary production (NPP) in the world's forests. These ecosystems play a large role in the world's carbon budget, and their dynamics, which are likely to be responding to global changes in climate and atmospheric composition, have major economic implications and impacts on global biodiversity. Although there is a long history of forest NPP studies in the ecological literature, current understanding of ecosystem-level production remains limited. Forest NPP cannot be directly measured; it must be approached by indirect methods. To date, field measurements have been largely restricted to a few aspects of NPP; methods are still lacking for field assessment of others, and past studies have involved confusion about the types of measurements needed. As a result, existing field-based estimates of forest NPP are likely to be significant underestimates.

In this paper we provide a conceptual framework to guide efforts toward improved estimates of forest NPP. We define the quantity NPP* as the summed classes of organic material that should be measured or estimated in field studies for an estimate of total NPP. We discuss the above- and belowground components of NPP* and the available methods for measuring them in the field. We then assess the implications of the limitations of past studies for current understanding of NPP in forest ecosystems, discuss how field NPP* measurements can be used to complement tower-based studies of forest carbon flux, and recommend design criteria for future field studies of forest NPP.